The present invention relates, in general, to a direction finding method and apparatus that can be used to locate the direction of a transmitter from a receiver located in space, on an aircraft, or on a fixed platform.
There are a number of known techniques for locating the position of a transmitter. Once such technique employs a plurality of fixed or rotating directional antennas which find the direction in space where an incoming signal from the transmitter is the strongest, or conversely, where a null output results that is caused by the destructive interference between signals. The simplest of these devices are those that consist of one more directional antennas that can be oriented to provide the greatest signal. Another simple system consists of an antenna pair that is separated by a fixed baseline so that when the incoming signal is incident at specific angles to the baseline, the voltage induced in one antenna is equal and opposite to that induced in the other. The output indication is the voltage sum, and a null output indicates alignment of the baseline with the signal. A disadvantage to these devices is that multiple antenna pairs arranged at crossed axes must be employed to eliminate ambiguities, and permit determination of both the elevation and azimuth angles of the incoming signal.
Another type of direction finding device is an interferometer. These devices employ two or more fixed antennas, and determine the direction of an incoming signal by measuring the phase differences between the signals arriving at the several antennas. Although these devices provide accurate measurements, a single axis interferometer can only measure the angle of signal arrival relative to the interferometer axis. In addition, precision direction finding requires baselines that are longer than one-half wavelength, which implies that ambiguities exist in assigning phase difference to angles of arrival. To resolve these ambiguities, a number of baselines (usually collinear) are required, and as with the previously discussed class of devices, in order to get a two dimensional indication of transmitter position, multiple, or crossed, interferometer axes must be used.
A third type of direction finding devices makes use of the Doppler shift. These devices measure the frequency shift in the received signal. Since the Doppler shift is dependent on the component of the relative velocity between the receiver and the transmitter along the line-of-sight, a single Doppler shift measurement can only determine the angle of arrival relative to the relative velocity vector. An advantage of using the Doppler technique is that the Doppler shift can be assigned to angle of arrival in an ambiguity free manner. A disadvantage of the Doppler technique is that unknown offsets in the transmitter frequency can be misinterpreted as Doppler shifts leading to misplacement of the transmitter. To mitigate this problem as well as to achieve a two-dimensional fix on a transmitter, a transmitter can be multiply observed while the transmitter-receiver geometry varies as, for example, during a satellite overpass. Multiple observations can then be used to solve for the unknown transmitter frequency as well as for angles with respect to several different relative velocity vectors which locate the transmitter. This is the technique used by the SARSAT search and rescue satellite system. Its disadvantage is that it requires multiple observations over a long period of time.
The present invention seeks to overcome the disadvantages of these known direction finding techniques.